High strength has been required for oil well pipes because recently oil wells have become deeper and deeper. That is, the oil well pipe of 110 ksi class has been recently used in many cases, instead of 80 ksi class and 95 ksi class pipes that were conventionally used widely for the oil well pipes. The 110 ksi class means a pipe having a yield stress (YS) of 110 to 125 ksi (758 to 862 MPa), while the 80 ksi class means a pipe having a YS of 80 to 95 ksi (551 to 654 MPa), and the 95 ksi class means a pipe having a YS of 95 to 110 ksi (654 to 758 MPa).
On the other hand, the oil well and gas well, which are developed nowadays, often contains corrosive hydrogen sulfide. In such environment hydrogen embrittlement, which is referred to as sulfide stress cracking, hereinafter abbreviated as SSC, is generated in the high strength steel and causes destruction. Accordingly, the most important issue for the oil well pipes of high strength is to overcome the SSC.
Techniques such as “making the steel extremely clean” and “grain refining” have been widely used as a method for improving the SSC resistance of the oil well pipe of the YS 95 to 110 ksi class (654 to 758 MPa class). For example, a method for reducing impurity elements such as Mn and P, in order to improve the SSC resistance, is disclosed in Patent Document 1. A method for improving the SSC resistance by double quenching in order to refine the crystal grain is disclosed in Patent Document 2.
Furthermore, the high strength oil well pipe such as 125 ksi class, which has not been applied for heretofore, has been examined recently. The 125 ksi class has a YS of 125 to 140 ksi, that is 862 to 965 MPa. Since the SSC is easily generated in the high strength steel, the further improvement of the material is required compared with the conventional oil well pipe of 95 to 110 ksi class (654 to 758 MPa class).
A method for providing a steel of 125 ksi class (862 MPa class) having a refined structure and excellent SSC resistance is disclosed in Patent Document 3. In this method a heat treatment, using induction heating, is applied. A method for producing a steel pipe using a direct quenching method is disclosed in Patent Document 4. The method provides the steel pipe of 110 to 140 ksi class (758 to 965 MPa class) which has excellent SSC resistance. In this method, the excellent SSC resistance can be attained by quenching from a high temperature in order to increase the martensite ratio, sufficiently dissolving alloy elements such as Nb and V during quenching, utilizing the elements for precipitation strengthening during the following tempering, and raising the tempering temperature.
An invention for optimizing alloy components in order to produce a low alloy steel having excellent SSC resistance of 110 to 140 ksi class (758 to 965 MPa class) is disclosed in Patent Document 5. Methods for controlling the form of carbide in order to improve the SSC resistance of a low alloy steel for an oil well of 110 to 140 ksi class (758 to 965 MPa class) are disclosed in Patent Document 6, Patent Document 7 and Patent Document 8. A technique for introducing precipitation of a great amount of fine V carbides in order to delay the generating time of the SSC of a steel product of 110 to 125 ksi class (758 to 862 MPa class) is disclosed in Patent Document 9.    Patent Document 1: Publication of Unexamined Patent Application Sho 62-253720.    Patent Document 2: Publication of Unexamined Patent Application Sho 59-232220.    Patent Document 3: Publication of Unexamined Patent Application Hei 6-322478    Patent Document 4: Publication of Unexamined Patent Application Hei 8-311551    Patent Document 5: Publication of Unexamined Patent Application Hei 11-335731    Patent Document 6: Publication of Unexamined Patent Application 2000-178682    Patent Document 7: Publication of Unexamined Patent Application 2000-256783    Patent Document 8: Publication of Unexamined Patent Application 2000-297344    Patent Document 9: Publication of Unexamined Patent Application 2000-119798